Switching high current and high voltage is required in some applications in military, medical, and commercial devices and systems. Depending upon on the application, a switching device may be required to switch tens of kilovolts and tens of kiloamperes. Devices for switching such high current and high voltage have been proposed to include a plurality of levels connected in series, each level having a plurality of high-power semiconductor switching devices connected in parallel. Each semiconductor includes a gate and a driver of the gate. To control these devices, the gate driver has special requirements such as high voltage isolation between levels, minimum delay time in gate pulses from one level to another, overvoltage protection, and sharing voltage protection.
Many drive circuits use pulse transformers in the gate circuitry to isolate one level of the switch from another. Because the switch comprises many semiconductors in series and in parallel, these pulse transformers become very large, costly, and impractical. In addition pulse transformers must be shielded to avoid external magnetic field pick up which could create unwanted low level gate pulses and, as a result, cause misfiring, which may destroy devices connected to the switch.
In response, methods have been developed using power stored in a capacitor floating with the device for the trigger energy. These methods use low-power triggers for a low-power solid state device that discharges the capacitor into the gate of the high-power semiconductor switching device. While still requiring a pulse transformer, because of the lower energy requirements in the gate circuitry, the switching device can be smaller. General examples of these switching devices are described in U.S. Pat. Nos. 5,444,610 and 5,646,833.
Other methods for switching the high-power semiconductor switching devices have been proposed. U.S. Pat. Nos. 6,396,672 and 6,710,994 describe a power electronic switch circuit that includes a silicon-controlled rectifier and a gate trigger circuit coupled to the gate of the silicon-controlled rectifier (SCR). A snubber capacitor is coupled to the anode and cathode of the SCR. Energy stored in the snubber capacitor provides the necessary energy to power the gate trigger circuit to trigger the SCR.
U.S. Pat. No. 6,624,684 describes a compact method for triggering thyristors connected in series using energy stored in a pulse forming network coupled to the gate of each thyristor. Each pulse forming network is coupled to a snubber circuit that, together with the pulse forming network, acts as a snubber capacitor to limit the dv/dt imposed on the thyristor, thereby preventing spurious turn-on of the thyristor. The pulse forming network provides current to the gate of the thyristor through a gate switch to turn on the thyristor while the snubber circuit provides a source of fast rising current to the anode of the thyristor to speed up turn-on as it discharges through the anode of the thyristor. Either a low-power electrical signal through a pulse transformer or an optical signal can be used to trigger the gate switch.
U.S. Pat. Nos. 5,933,335 and 5,180,963 provide examples of an optically triggered switch. In U.S. Pat. No. 5,180,963, there is described an optically triggered solid state switch. The switch uses an optical signal for each set of two high power solid state devices. The optical signal triggers a phototransistor, which in turn triggers a low power solid state device. The low power solid state device then discharge a capacitor through a pulse transformer, producing signals in the gates of two high power solid state devices to turn on the devices.
U.S. Pat. No. 7,072,196 describes a method of turning on a high voltage solid state switch that comprises a set of solid state devices, such as thyristors, connected in series. The switch comprises a snubber circuit coupled to the anode and cathode of each solid state device to speed up turn-on.